A gas turbine for driving an electrical generator, compressor, or the like can be exhausted to a waste heat boiler to recover heat for steam generation and also provide combustion aid for firing additional oil or gas fuel to increase the steam generating capacity of the waste heat boiler.
A conventional forced draught fan is provided to supplement the combustion air supplied by the gas turbine and also to allow the waste boiler to operate as a conventional boiler when the gas turbine is not operating. The fuel burner or burners used, however, have to be designed to achieve efficient combustion when using either the very hot (900.degree. to 1000.degree. F say) turbine exhaust gas or the cool (80.degree. F say) air supplied by the forced draught fan. To achieve this a double skin windbox with special burners having two quarls can be employed or the gas turbine exhaust can be introduced through apertures in the furnace adjacent to the fuel burners.
The use of a double skin windbox and these special burners leads to design and construction complications which it is desirable to avoid. In addition it is in the interests of efficiency to use the turbine exhaust gases as part of the combustion air for the burner or burners.